Are there solid objects that do not emit visible light?
To my limited and humble knowledge, creating such an object would require us to "only" shift the spectrum of emitted radiation. Is it possible? If yes, how?
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3$\begingroup$ Cold objects usually do not emit visible light... $\endgroup$– CuriousOneCommented May 5, 2016 at 6:03
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5$\begingroup$ Do you mean reflect visible light? All materials emit EM radiation due to black body emission, but at normal temperatures the amount of visible light emitted is negligibly small. $\endgroup$– John RennieCommented May 5, 2016 at 6:07
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$\begingroup$ Good catch. Haven't thought it through really. Those stupid morning thoughts.. :) Thank you all for your response. $\endgroup$– sjaustirniCommented May 5, 2016 at 18:03
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1$\begingroup$ Do perfectly reflecting bodies radiate? $\endgroup$– Keith McClaryCommented May 6, 2016 at 5:14
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1 Answer
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The microscopic mechanism of emitting photon in a solid is the transformation of kinetic energy of atoms into EM energy. If an atom is in an excited state due to collisions among other atoms, then it will emit photon when it jumps into the ground state, and the energy of the photon is $$ E=\varepsilon(\text{excited state})-\varepsilon(\text{ground state})\sim k_{B}T $$
The energy of the photon of visible light is about $2\sim3\,\mathrm{eV}$. So if the temperature $k_{B}T\ll2\,\mathrm{eV}$, then the solid is difficult to emit visible light. The normal indoor temperature is $298\,\mathrm{K}$, i.e. $k_{B}T\sim0.025\,\mathrm{eV}$, so it is difficult to emit visible light in a solid in normal indoor temperature.
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$\begingroup$ So the emission happens irrespective if a body is in equilibrium with its environment. But if it is in equilibrium the heat that goes to the environment must be equal to the heat that goes into the system from the environment. In that heat we should include radiation? $\endgroup$– AntonCommented Sep 11, 2021 at 17:46